Do not return probe failure with missing DMA resources, allow driver
to work in PIO mode.  Tested on da850-evm by mounting partition
followed by file creation and deletion.
Signed-off-by: NManjunathappa, Prakash <prakash.pm@ti.com>
Tested-by: NSekhar Nori <nsekhar@ti.com>
Signed-off-by: NChris Ball <cjb@laptop.org>